Doctors and patients have long assumed that opioids are uniquely powerful medicines for chronic pain, despite their risks. But it turns out that this reputation may be a myth.

A government-funded study published Tuesday is among the first long-term studies to compare opioids like oxycodone and morphine to common painkillers such as acetaminophen in patients with chronic back pain and arthritis, researchers said.

After a year of treatment, opioids weren't any better at improving pain related to daily functioning, such as ability to sleep and work, the study found. The opioids were also slightly inferior at controlling pain intensity, and patients on them reported many more side effects, according to the results published in the Journal of the American Medical Association.

"The fact that opioids did worse is really pretty astounding," said Roger Chou, an internist at Oregon Health & Science University and a co-author of Centers for Disease Control and Prevention guidelines on opioid use for chronic pain, who was not involved in the recent study. "It calls into question our beliefs about the benefits of opioids."

The findings run counter to years of medical practice in the U.S., where more so than in other countries, opioids have been prescribed to millions of patients for chronic pain over the years -- even though data on their long-term effectiveness was lacking. While doctors are pulling back now, a surge in opioid use and abuse has led to an overdose crisis that kills tens of thousands of Americans each year.

The study is also another blow for opioid manufacturers, such as Purdue Pharma LP, that are already facing hundreds of lawsuits filed by U.S. cities and counties over their role in the opioid crisis. More than 300 lawsuits are on hold as the drugmakers engage in settlement talks ordered by the federal judge in Cleveland who is overseeing the litigation.

In the study, lead author Erin Krebs and her colleagues at the Minneapolis Veterans Affairs Health Care System randomly assigned 240 patients with chronic back pain, or hip or knee arthritis, to be treated either with opioid painkillers like morphine and oxycodone, or nonopioid medicines including standard anti-inflammatory drugs like naproxen, or topical analgesics such as lidocaine.

After a year, about 60 percent of patients in each group experienced significant improvements in their ability to perform daily functions without pain interfering. But pain intensity improved significantly in just 41 percent of patients in the opioid group, compared with 54 percent in the nonopioid group, said Krebs, a primary care doctor at the Minneapolis VA. And patients taking opioids reported experiencing twice as many side effects.

If anything the study should have been biased in favor of the opioids, researchers said, because patients knew what drugs they were getting, and many went into the study believing that opioids were better.

The results "will be surprising for a lot of people," Krebs said. "Opioids have this reputation as powerful painkillers and I don't think it is well deserved, at least for chronic pain."

While opioids provide potent relief for acute pain, that doesn't necessarily translate to a chronic pain situation, where the pain often becomes disassociated from the original injury. Long-term studies haven't been required for regulatory approval of their sales, and drugmakers have had no incentive do any.

The 2016 CDC chronic pain guidelines co-authored by Chou, the Oregon university internist, found that most controlled studies of opioids lasted less than six weeks. The government agency concluded there was "insufficient evidence" for their long-term benefits, but plenty of evidence for harm.

"This the first randomized trial that demonstrates you do not need opioids for these common chronic pain conditions, that common drugs are as good," said Gary Franklin, a neurologist and occupational health researcher at the University of Washington who did early studies linking prescription opioid use to fatal overdoses. "That is why this is so important."